C.S. Lewis's nickname among family and close friends was Jack.
Lewis began using “Jack” after a childhood incident involving his dog, Jacksie. When the dog was killed, Lewis reportedly said he would henceforth call himself Jacksie, and the nickname later developed into Jacks or Jack. “Jack” became the name used by relatives, friends, and many of his academic associates.
The name can cause confusion because Lewis published under the initials C.S. Lewis and was born Clive Staples Lewis. “Jack” was not a pen name for his books; it was his everyday personal name. His brother Warren was commonly called Warnie.
The nickname also appears in biographies and collections of Lewis’s letters, where friends often address him as Jack. Readers may encounter “C.S. Lewis” in bibliographies and “Jack” in memoirs describing his private life.